This application relates generally to optical packaging systems and methods. More specifically, this application relates to optical packaging and circuit fabrication systems and methods using three-dimensional, direct-write lithography.
Present optical packaging systems are a direct descendant of gun sights developed in WW II in which optics are manually aligned and then epoxied to machined metal scaffolds. The results are typically unreliable, large, expensive and of limited complexity due to the moderate individual yield. Traditional planar lightwave circuits can fabricate complex optical interconnections but are generally unable to incorporate hybrid subcomponents and thus can only implement devices encompassed by a single material system. An alternative approach is to reduce the scale of the “scaffold” via lithographic silicon micro-machining, but the precise and identical nature of each silicon bench demands similar tolerances for the subcomponents that are not economically feasible.
The use of 3D direct-write lithography to form localized permanent index changes in glass with femto-second lasers was first published in 1996. Since then, there have been demonstrations of optical waveguides written both parallel and perpendicular to the writing light, and beam shaping to make perpendicular waveguides with symmetric profiles. There have also been demonstrations that take advantage of the full three-dimensional (3D) capabilities of the direct-write system such as a 1×3 splitter, 3-waveguide directional coupler, and a microring resonator. One feature of this method is that the extremely high power of the ultra-short pulsed laser induces nonlinear absorption at the focus in order to locally change the properties of the glass.
Planar (2D) direct write lithography is well known as a method to form planar waveguides. By employing a material with linear sensitivity, such as photopolymer, the laser power requirements can be reduced sufficiently to enable continuous (not pulsed) sources, producing naturally smooth guides. The polymers used are related to photoresists in that the laser polymerizes the thin (order several microns) monomer layer at focus and a subsequent solvent wash removes all unexposed material. An optional overcoat of a cladding material completes the process. This material process is not compatible with deeply buried, 3D structures.
The Dupont® company commercialized the use of a sensitive volume photopolymer in place of glass for use in holography. Such materials require no wet processing step. Dupont® also applied this material to planar (2D) optical waveguide fabrication and to laminated 3D circuits. By cutting slots in 2D, lithographically-formed planar polymer waveguides, one can insert thin optical components such as Faraday rotators to implement hybrid optical circuits. These slots are filled with liquid monomer and cured in place to reduce reflection losses. Only limited functionality can be attained by this method since the hybridized components must be thin (to minimize diffraction losses within the part) and optically flat to much less than a wavelength (to minimize coupling losses). More traditional hybridization methods use silicon etching to make miniature optical “benches” to hold precisely fabricated (i.e. expensive) hybrids to silicon waveguides. Similar methods have recently been applied to planar lithographic polymer waveguides for optical back-plane interconnects including the encapsulation of a vertical cavity laser in the polymer.
More recently, Toyota® has embedded a plastic optical fiber and a wavelength multiplexing filter into a thick polymer in order to make a cheap, robust component for use in autos. The polymer was a traditional expose-and-rinse material, not the self-developing polymer discussed above, limiting the guides to be large and multi-mode. In this example, a multi-mode waveguide was written in the photopolymer by embedding a multi-mode plastic optical fiber into the material, then letting light from the fiber “self-write” a waveguide by locally polymerizing the material, thus creating a weak lens, and propagating light through the material in a soliton-like fashion. This technique has limited control over the shape and direction of the resulting waveguides, and single mode operation has not yet been demonstrated over a significant distance. Very little to no control over the waveguide shape, size or direction is given with this method.